1. Field of the Invention
The present invention relates to devices for determining optical aberrations and more particularly to wavefront sensors for determining the optical path or phase errors present on an input beam.
2. Description of Related Art
Wavefront slope sensors are used, for example, in adaptive optical systems to determine the adjustments necessary to compensate for wavefront distortions produced by the optical train and/or the intervening medium. In most applications, the ideal is a zero wavefront slope over the entire cross section of the beam. A non zero slope indicates distortion and dispersion of the beam, the result being a diminished power delivery or degraded image producing capability.
An outgoing beam can be pre-distorted for example, by varying the pressure applied across the deformable primary mirror as a function of the measured distribution. In this way, an appropriate slope related parameter can be minimized to enhance either the power delivered or the quality of the imagery received.
The criteria according to which wavefront slopes sensors are evaluated include reliability, speed, resolution, and cost. Another primary consideration is the flexibility for use in both pulsed and continuous wave modes.
Several of the available or described slopes sensors include moving elements. The movement may be used to sample different parts of the beam, or to provide multiple measurements of a single part of the beam as necessary to compute the slope. Such systems are disadvantageous in that they are generally limited to continuous wave applications. Also the time involved in the mechanical movement limits the measurement speed of the system. This is particularly problematic where real time adjustments are required. Furthermore, the incorporation of moving parts reduces the reliability and increases the cost of the system.
There is another class of wavefront slope sensors which use multiple subapertures in sampling the wavefront. For example, some Hartman-type sensors use a grating which defines multiple subapertures. The average slope within each subaperture is determined, and the collective determination provides a slope distribution beam.
A major problem with the grating sensor is a difficulty of manufacturing high resolution gratings. Additionally, a significant percentage of the incident light can be lost by reflection or scattering at the grating. Also, diffraction affects become more severe at smaller apertures. Finally, it turns out that the mechanical aperture sensors have greater computational requirements by about a factor of three to four. Thus, real time performance is impaired. The above disclosed available sensors have provided a choice between the limitation of the systems with mechanical movement and those with mechanical grids or subapertures. Thus, there is a need for a reliable, cost effective, fast wavefront sensing which efficiently processes data, provides for high resolution sensing, and is applicable to pulse as well as continuous wave applications.